310 CHAPTER 12brothers, related to her by r = 0.25). But the queen gains more inclusive fitness
through her own sons (related to her by r = 0.5) than through her daughters’ sons
(related to her by r = 0.25). Queens of many species therefore destroy their workers’
eggs, and in some species (including honeybees, Apis mellifera) workers destroy
the eggs of other workers [64, 78]. This is one of the best examples of policing of
noncooperators in social species, and it illustrates that kin selection can underlie
the evolution not only of altruism, but also of selfishness.
There is also conflict between queens and workers about how many males and
reproductive females the colony should produce. A queen’s fitness is maximized by
a 1:1 sex ratio, since she is equally related to her daughters and sons. But workers can
control the sex ratio among the larvae destined to be males or queens, by feeding
some more than others, or even by killing some of them. In a colony with a single
queen, a worker’s inclusive fitness is maximized by rearing young queens and males
in a 3:1 ratio, because a worker is related by r = 0.75 to her queen sisters, but only by
r = 0.25 to her brothers. In colonies with multiple queens, however, workers should
favor a sex ratio closer to 1:1 (because not all females are full sisters and so r < 0.75).
Data support these predictions [18, 61]. Within some species of ants, wasps, and
bees, some colonies have a single queen that mated with a single male, and others
have colonies with either multiple queens or a single queen that mated with multiple
males. A review of studies of species with this kind of variation found that the pre-
diction about sex ratio was upheld in 18 of the 19 species [61].l evels of Selection
The basic principle of evolution by natural selection is simple: the entities that make
more copies of themselves increase in frequency through time. Usually, the “enti-
ties” in question are alleles. But the same principle applies to anything that can rep-
licate—bits of DNA, mitochondria, entire chromosomes, even groups of individuals.
In this section we will see how Darwin’s concept of natural selection can be applied
at these different levels to understand important features of the natural world.Selfish DNA
Meiosis is a remarkably democratic affair: the two alleles carried by an individual
at a locus usually have an equal chance of being passed to an offspring. But con-
sider a mutation that can tilt the odds in its favor so that its chances are greater
than 50 percent. Any mutation that can do so will enjoy an evolutionary advan-
tage, a situation called segregation distortion. It can spread in the population even
if it actually decreases survival or reproduction. Given the strong evolutionary
incentive to cheat at inheritance, it is surprising how fair meiosis usually is.
There are, however, mutations that do cheat [47b]. Meiosis is fundamentally dif-
ferent in males and females, and so the ways that mutations are able to break Men-
del’s laws are quite different in the two sexes (FIGURE 12.11). Alleles that cause
segregation distortion in males are known from diverse groups, including mam-
mals, insects, and fungi. Some alleles at the t locus in the house mouse (Mus mus-
culus) are transmitted to about 95 percent of a heterozygous male’s sperm. Sperm
that carry of these alleles gain an advantage by secreting a toxin that kills other
sperm in the testes. These sperm are themselves immune to the toxin because
they carry a resistance allele at a second locus. This transmission advantage causes
the selfish alleles at the t locus to spread, even though they reduce the fertility of
males. A killer allele must be inherited together with the resistance allele, or else
sperm with the killer allele will commit gametic suicide. That explains why in mice
and other species with this kind of segregation distortion, the two loci are always
tightly linked, and are often found in regions of the genome with reduced recom-
bination (such as the sex chromosomes).12_EVOL4E_CH12.indd 310 3/22/17 2:39 PM